The present invention relates to a misfire discriminating method for an engine wherein a misfire is detected from the variation in the rotation of an engine.
In general, it is ideal for producing a stable output that combustion in a multicylinder engine undergoes the same process every cycle. In the multicylinder engine, however, the combustion is liable to fluctuate for such reasons as listed below:
(1) Nonuniformity in the distribution rate of intake air attributed to the complication of the shape of an intake pipe, the interferences of the intake air among cylinders, etc. PA0 (2) Some differences among the combustion temperatures of the individual cylinders attributed to cooling routes. PA0 (3) Manufactural scattering in the volumes of the combustion chambers of the individual cylinders, the shapes of pistons, etc.
Heretofore, the so-called combustion fluctuation has been suppressed to the minimum by the air fuel ratio controls and ignition timing controls of the individual cylinders. In a recent high-performance engine tending toward a higher output and a lower fuel cost, however, when any of the injectors, ignition plugs etc. has deteriorated or broken down, intermittent misfire arises and incurs lowering in the output.
In general, a misfire of a cylinder can be detected by detecting a variation in the rotation of the engine due to the misfire and comparing the variation in the rotation with a predetermined discriminating level. For example, according to the official gazette of Japanese Patent Application Laid-open No. 118031/1987, there is disclosed a technique for discriminating an abnormal combustion cylinder based on a maximum value of a variation in the rotation of an engine and a counted value of pulse signals by measuring an interval of a plurality of pulse signals generated at each one revolution of a crankshaft, and discriminating the maximum value of the variation in the rotation of the engine from a timing change of the pulse interval.
Further, according to the official gazette of Japanese Patent application Laid-open No. 112646/1990, there is disclosed a technique for detecting an abnormal cylinder from a variation in the momentary rotation of each cylinder by detecting a plurality of angular positions per one revolution of a multicylinder internal combustion engine, and detecting the momentary rotation of a special rotating position of each cylinder from an interval of the detected angular positions.
However, a variation in the rotation due to a cause except the misfire might occur in the engine. For example, when a variation in the rotation occurs continuously for a predetermined time due to a snatch, it is difficult to distinguish the variation in the rotation due to the snatch from the variation in the rotation due to the misfire, and an erroneous discrimination might occur merely by comparing the variation in the rotation of the engine with the misfire discrimination level.
In addition, in order to normally detect the rotating speed of the engine, it is, for example, necessary to provide a rotor to be rotated in cooperation with a crank shaft driver by the engine and rotation detecting means composed of a sensor for detecting a predetermined position corresponding to a cranking position of the rotor. A small mounting position error exists at the rotor and the sensor for constituting the rotation detecting means in each engine, and allowable errors in manufacture of the respective components exist.
Therefore, the variation in the rotating speed of the engine to be detected by the rotation detecting means includes the above-described error different at each engine. If the misfire is discriminated according to a unitarily set discrimination level, an erroneous discrimination might occur.
Moreover, a relatively small variation in the rotation of the engine might occur due to a cause except the misfire such as a stepwise difference due to acceleration/deceleration. If a misfire once occurs, continuous misfires might occur among the cylinders. In such a case, if the misfire is discriminated merely by comparing the variation in the rotation of the engine with a misfire discrimination level, the variation in the rotation due to nonmisfire mightly erroneously discriminated as the misfire, or, on the contrary, the continuous misfire might be erroneously discriminated as the mere variation in the rotation, and hence it is difficult to accurately detect the misfire.
Even when the intermittent misfire has occurred in one cylinder in the multicylinder engine, a driver often drives an automobile without noticing the misfire. Besides, it is difficult to diagnose during the drive whether the cause of the misfire is temporary or is the deterioration of any of the injectors, the ignition plugs etc.
Therefore, according to the official gazette of Japanese Patent Application Laid-open No. 258955/1986 by way of example, a comparison is made between the difference of the minimum value and maximum value of the r.p.m. of an engine corresponding to a cylinder at the previous combustion stroke and the difference of the minimum value and maximum value of the engine r.p.m. corresponding to the cylinder at the present combustion stroke. The combustion condition of the pertinent cylinder is discriminated, depending upon whether or not the discrepancy between the compared values falls within a preset reference value. In a case where abnormal combustion has occurred in excess of a predetermined number of times, a misfire is judged, and warning is given.
With the prior-art technique mentioned above, the misfire condition is discriminated by grasping the revolution fluctuation of the engine. However, when the misfire is diagnosed at the cranking of the engine or at the initial stage of firing start at which the revolution fluctuation is great, the diagnosis is liable to become erroneous and poses a problem in reliability.